Sulfonation is an important process for manufacturing anionic surfactants. Fatty acid derivatives, particularly fatty esters, are among many different starting materials that can be converted to valuable surfactants. For instance, a mixture of saturated and unsaturated fatty esters can be sulfonated with sulfur trioxide, then neutralized, to give an anionic surfactant useful in detergents (see, e.g., U.S. Pat. Nos. 4,545,939 and 4,816,188). Natural oils can be sulfonated (see, e.g., U.S. Pat. No. 5,319,117), but it is more common to use alkyl esters of fatty acids as starting materials. Saturated fatty methyl esters are frequently sulfonated to produce α-sulfonated methyl esters (see, e.g., U.S. Pat. Nos. 5,475,134; 5,945,394, and 5,616,781), which are used in laundry detergents (U.S. Pat. No. 7,820,612), soaps (U.S. Pat. No. 6,172,026), and light-duty liquid detergents (U.S. Pat. Nos. 5,118,440 and 5,637,758). Sulfo-estolides (see, e.g., U.S. Pat. Nos. 7,666,828 and 7,879,790 and U.S. Pat. Appl. Publ. No. 2010/0016198) are a particular class of sulfonated fatty esters that have value in laundry detergents, hard surface cleaners, and personal care applications.
The fatty acids or esters used to make these sulfonates and their derivatives are usually made by hydrolysis or transesterification of triglycerides, which are typically animal or vegetable fats. Consequently, the fatty portion of the acid or ester will typically have 6-22 carbons with a mixture of saturated and internally unsaturated chains. Depending on source, the fatty acid or ester often has a preponderance of C16 to C22 component. For instance, methanolysis of soybean oil provides the saturated methyl esters of palmitic (C16) and stearic (C18) acids and the unsaturated methyl esters of oleic (C18 mono-unsaturated), linoleic (C18 di-unsaturated), and α-linolenic (C18 tri-unsaturated) acids. The unsaturation in these acids has either exclusively or predominantly cis-configuration.
Recent improvements in metathesis catalysts (see J. C. Mol, Green Chem. 4 (2002) 5) provide an opportunity to generate reduced chain length, monounsaturated feedstocks, which are valuable for making detergents and surfactants, from C16 to C22-rich natural oils such as soybean oil or palm oil. Soybean oil and palm oil can be more economical than, for example, coconut oil, which is a traditional starting material for making detergents. As Professor Mol explains, metathesis relies on conversion of olefins into new products by rupture and reformation of carbon-carbon double bonds mediated by transition metal carbene complexes. Self-metathesis of an unsaturated fatty ester can provide an equilibrium mixture of starting material, an internally unsaturated hydrocarbon, and an unsaturated diester. For instance, methyl oleate (methyl cis-9-octadecenoate) is partially converted to 9-octadecene and dimethyl 9-octadecene-1,18-dioate, with both products consisting predominantly of the trans-isomer. Metathesis effectively isomerizes the cis-double bond of methyl oleate to give an equilibrium mixture of cis- and trans-isomers in both the “unconverted” starting material and the metathesis products, with the trans-isomers predominating.
Cross-metathesis of unsaturated fatty esters with olefins generates new olefins and new unsaturated esters that can have reduced chain length and that may be difficult to make otherwise. For instance, cross-metathesis of methyl oleate and 3-hexene provides 3-dodecene and methyl 9-dodecenoate (see also U.S. Pat. No. 4,545,941). Terminal olefins are particularly desirable synthetic targets, and Elevance Renewable Sciences, Inc. recently described an improved way to prepare them by cross-metathesis of an internal olefin and an α-olefin in the presence of a ruthenium alkylidene catalyst (see U.S. Pat. Appl. Publ. No. 2010/0145086). A variety of cross-metathesis reactions involving an α-olefin and an unsaturated fatty ester (as the internal olefin source) are described. Thus, for example, reaction of soybean oil with propylene followed by hydrolysis gives, among other things, 1-decene, 2-undecenes, 9-decenoic acid, and 9-undecenoic acid. Despite the availability (from cross-metathesis of natural oils and olefins) of unsaturated fatty esters having reduced chain length and/or predominantly trans-configuration of the unsaturation, fatty acid and ester sulfonates made from these feedstocks appear to be unknown. Moreover, sulfonates have not been made from the C18 unsaturated diesters that can be made readily by self-metathesis of a natural oil.
In sum, traditional sources of fatty acids and esters used for making fatty ester sulfonates generally have predominantly (or exclusively) cis-isomers and lack relatively short-chain (e.g., C10 or C12) unsaturated fatty portions. Metathesis chemistry provides an opportunity to generate precursors having shorter chains and mostly trans-isomers, which could impart improved performance when the precursors are converted to downstream compositions (e.g., in surfactants). New C18 difunctional fatty ester sulfonates are also potentially available from oil or C10 unsaturated acid or ester self-metathesis.